Referring to FIG. 1, integrated Circuit RF/Microwave amplifiers are typically constructed of multiple amplification stages 10, 11 and 12 connected in series on a single semiconductor die 05. First stage 10 connects to a signal source though an input impedance transformation network (not shown) that may be fabricated on-chip or off-chip, and last stage 12 connects to a load impedance through an output impedance transformation network (not shown) that also may be fabricated on-chip or off-chip. For amplifiers that have three or more stages, there will be one or more amplifier stages (for example stage 11) that are terminated on both their input and output by another amplifier stage. This applies to any amplification stage located between first 10 and last stage 12. The networks connecting these middle stages are referred to as inter-stage transformation networks.
The design and performance of inter-stage transformation networks is a major contributor to overall amplifier performance. Of particular interest for this invention is the role that the inter-stage networks play in preventing unwanted oscillations from occurring in the amplifier. The complexity of the design of inner-stage networks is reduced in many semiconductor technologies due to the presence of through-substrate vias. A through-substrate via is a path that penetrates the top semiconductor substrate surface where the active (and passive) devices are fabricated and connects directly to the bottom semiconductor substrate surface which is a metalized ground plane. The prevalent technology in the field of RF/Microwave amplifiers with through-substrate vias is Gallium Arsenide (GaAs). For technologies such as Si and SiGe, through-substrate vias are not an option since there is no bottom ground plane.
When developing RF/Microwave amplifiers in technologies that lack through-substrate vias, grounding becomes an important and difficult problem. The only means for connecting a Si amplifier circuit to ground is through the use of wirebonds that go between the chip and the package ground plane. The parasitic inductance from these wirebonds introduces a significant amount of inductance between the amplifier circuit and ground.
A common method for overcoming grounding problems is to utilize a differential amplifier topology. As one skilled in the art will understand, a differential amplifier topology allows for a virtual ground to be generated on-chip that will be independent of the amount of wirebond inductance present. By making use of this virtual ground, a high-performance differential circuit may be designed. Any input signal to a differential amplifier may be separated into differential-mode and common-mode components. As the name implies, the differential-mode signal is the primary mode of operation for the amplifier. However the common-mode performance of the amplifier must also be considered to ensure proper amplifier operation.
The virtual ground created in a differential amplifier is only present for differential-mode signals. All of the parasitic wirebond inductance is present for common-mode signals and presents numerous problems. Of particular concern is the reduced stability of the amplifier in the common-mode. The primary cause of the reduced common-mode stability results from the inductance caused by wirebonds 13, 14, and 15. The large inductance associated with wirebond 14, which is in series with the emitters of transistors 43 and 44 can cause devices 43 and 44 to become unstable. This is not a concern for the differential-mode signal since RF current does not flow through the wirebonds as shown by the equivalent differential-mode circuit shown in FIG. 3.
Another approach to reduce the effects of the inductance associated with wirebond 14 is to reduce the common-mode gain in order to improve the active device's stability. This can be done by adding resistive loss to the inter-stage network that only affects the common-mode signal. This allows for the differential mode signal to be unaffected while helping to stabilize the common-mode.